1. Technical Field
The present invention relates to a distortion compensation amplification device having a pre-distorter (PD) that compensates for non-linear distortion generated in an amplifier using a pre-distortion method, and more particularly, to a distortion compensation amplification device having a pre-distorter that causes pre-distortion learning coefficients (for example, correspondence given by a function, which is acquired therefrom, that gives an inverse characteristic of a non-linear characteristic of the amplifier) to efficiently converge.
2. Background Art
In general, input and output characteristics of a power amplification unit are linear in a region in which an input level is low, and are non-linear in a region in which the input level is greater than a predetermined level, and the output power is finally saturated. Typically, since an operation point close to a saturation point is used to enhance power efficiency of the power amplification unit, non-linear distortion is caused by the non-linearity of the amplifier. Due to this non-linear distortion, unnecessary signal components leak inside a desired signal band and outside the desired signal band (adjacent channel).
In general, when time is represented by t, an input signal of an amplifier is represented by complex number z(t), and an output signal of the amplifier is represented by complex number y(t), Expression 1 is established.
In Expression 1, Gain represents a gain of the amplifier and is a real number. Dn is an n-th-order non-linear distortion coefficient generated in the amplifier and is a complex number.
In Expression 1, the input and output characteristics of the amplifier are expanded in a power series. Gain·z(t) which is the first term in the right side is a linear component (desired wave) and the second term or terms subsequent thereto in the right side are non-linear components (unnecessary waves).y(t)=Gain·z(t)+D3|z(t)|2z(t)+D5|z(t)|4z(t)+D7|z(t)|6z(t)+ . . .   Expression 1
The reason that Expression 1 includes only odd-order terms such as third-order, fifth-order, seventh-order, . . . is that a spectrum of odd-order distortion occurs in the vicinity of a spectrum of linear component Gain·z(t) as a frequency spectrum of the output signal of the amplifier is viewed. On the other hand, the even-order distortion appears as a difference-frequency component generated in a baseband and two or more times of a high-frequency component, and thus can be easily attenuated by a band-limiting filter (or a band-pass filter) and the like.
Particularly, since a base station is high in transmission power, the non-linear distortion is strictly prescribed in an ACLR (Adjacent Channel Leakage power Ratio), a spurious standard, a spectrum emission mask, or the like. Accordingly, it is an important problem how to reduce the non-linear distortion.
A pre-distortion method is known as a distortion compensation method of compensating for the non-linear distortion of the power amplification unit. The pre-distortion method is a method of compensating for the distortion generated in the power amplification unit by previously giving an inverse characteristic of AM-AM conversion and AM-PM conversion, which is the non-linear characteristics of the power amplification unit, to the input signal of the amplifier.
FIG. 11 shows a configuration example of an amplifier with a pre-distorter compensating for the distortion of a power amplification unit using the pre-distortion method. The input signal is also input to a controller 14 as necessary, as in the case where a waveform comparison method is used in the controller 14.
FIG. 2 shows a configuration example of a pre-distortion executing unit 13.
Processing units 1 to 7 and 11 to 14 shown in FIG. 11 are the same as shown in FIG. 1 which is referred to in an embodiment of the invention to be described later. FIG. 2 is referred to in an embodiment of the invention to be described later and is referred to here for the purpose of explanatory convenience, but is not intended to unnecessarily limit the invention.
Here, a waveform comparison method or a perturbation method using outside-band distorted power as an error function is used as an adaptive update algorithm of a distortion compensation table 12.
In the waveform comparison method, the controller 14 calculates an error signal from a feedback signal (an output signal of a power amplification unit 4 including distortion) acquired from an A/D converter 7 and an input signal (a signal input from the input side to the controller 14) and causes the details of the distortion compensation table 12 to converge using an LMS (Least Mean Square error) algorithm or the like.
The perturbation method is a method of using the outside-band power, which is obtained by performing Fourier transformation on the feedback signal, as an estimation function and selecting a coefficient with a smaller estimation function when the coefficient of the pre-distorter 1 varies, thereby causing the details of the distortion compensation table 12 to converge.
Both methods have different features. In the waveform comparison method, considerable precision is necessary to adjust a delay time or amplitude of the input signal and the output signal, thereby complicating the processes. On the other hand, the perturbation method uses only the feedback signal and thus can be embodied with relatively simple processes, but requires a long time for convergence.
First, problem 1 will be described.
The above-mentioned pre-distorter has room for improvement, in efficient convergence in pre-distortion information (for example, the details of the distortion compensation table 12), and requires higher efficiency.
In a memoryless pre-distorter as a specific example, since various orders of pre-distorter coefficients have an influence on each other, there is a problem in that much time is required for causing the pre-distortion information to adaptively converge.
Similarly, in a pre-distorter (a memory-effect pre-distorter) compensating for a memory effect, since various orders of pre-distorter coefficients have an influence on each other, there is a problem in that much time is required for causing the pre-distortion information to adaptively converge.
When both of the two pre-distorters are used, since the coefficients of the pre-distorters are independent of each other but have an influence on each other, there is also a problem in that much time is required for causing the pre-distortion information to adaptively converge.
To solve problem 1, as described in the embodiment of the invention to be described later, an effective method using a set of orthogonal polynomials including orthogonal functions as a function for giving an inverse characteristic of the non-linear characteristics of the amplifier so as to shorten the time of convergence is considered to be applied in a pre-distorter (for example, see Japanese Patent Application No. 2007-285032 filed by the present applicant).
Then, problem 2 will be described.
FIG. 12 shows a configuration example associated with an operation of a controller 101 (corresponding to the controller 14 shown in FIG. 11) according to the background art as a configurational example of the controller 101 when the waveform comparison method is used as an adaptive algorithm of a pre-distortion learning coefficient. FIG. 12 also shows an A/D converter 7 and a distortion compensation table 12.
Here, an update method when an adaptive algorithm of pre-distortion learning coefficients Ai and Bi is used will be described.
Functions of Φi, Ai, Bi, and E[·] will be described in an embodiment to be described later.
The controller 101 in this embodiment includes a subtractor 111 and an adaptive algorithm unit 112.
The subtractor 111 calculates a difference between the input signal (non-distorted signal) to the pre-distorter and the feedback signal (distorted signal) from the amplifier (the amplifier of the power amplification unit 4 in this embodiment) as an error signal e(t), by subtracting the input signal from the A/D converter 7 from the input signal to the pre-distorter. In this embodiment, a non-linear distortion component is the error signal e(t).
The adaptive algorithm unit 112 updates the pre-distortion learning coefficients (for example, the details in the distortion compensation table 12 acquired thereby) using algorithms shown in Expression 2 and Expression 3 using the LMS algorithm on the basis of the error signal e(t) acquired by the subtractor 111.Ai[t+1]=Ai[t]+μ(E[Φi*(t)e(t)]/E[|Φi(t)|2])  Expression 2Bi[t+1]=Bi[t]+μ(E[Φi*(t)e(t)]/E[|Φi(t)|2])  Expression 3
Here, 0<μ≦1 is established and the magnitude of an error is normalized using E[|Φi(t)|2] of the denominator. The error signal e(t) is expressed by Expression 4.
In Expression 4, x(t) represents the input signal to the pre-distorter and PAout represents the input signal to the controller 101, which is obtained by feeding back the output signal of the amplifier. Here, τ represents a temporal synchronization error and Gain′ represents an amplitude-adjusting coefficient based on the amplification rate of the amplifier.e(t)=x(t)−PAout(t−τ)/Gain′  Expression 4
In this case, the adjustment of the delay time and the level (amplitude) is important. When the distortion is completely compensated for, the error is e(t)=0. However, when a delay time difference exists (τ≠0) or when Gain′*x(t)≠PAout(t), the error is e(t)≠0 and thus the distortion seems to appear.
For example, since the ratio of the desired signal power and the distorted power is 30 to 60 [dB] which is very great, this adjustment is important. When it is intended to solve this problem, there is a problem in that complicated and precise calculations are required and thus the circuit scale increases.
In this way, in the above-mentioned configuration of the pre-distorter (for example, the configuration shown in FIG. 12), the configuration for causing the pre-distortion learning coefficients (for example, the details of the distortion compensation table 12 obtained thereby) to converge using an adaptive algorithm still has room for improvement, thereby requiring higher efficiency.
To solve problem 2, a technique of reducing a circuit scale because the complicated and precise operation is not required when the waveform comparison method is used as the adaptive algorithm by using the configuration shown in FIGS. 3(a) and 3(b) which are referred to by the embodiments of the invention to be described later is considered (for example, see Japanese Patent Application No. 2007-285032 filed by the present applicant).